This invention relates generally to the field of wind energy, and more particularly to vertical axis turbines and Turbines with more than one axis or shaft.
Wind turbines utilize a rotor for converting the energy of the air stream into rotary mechanical power as a power conversion device from the wind. Wind machines can take advantage of a free and inexhaustible power source of mechanical power for various purposes including driving an electrical generator. Most wind turbines are the horizontal-axis wind turbine (HAWT). However, many wind turbines are known as vertical-axis turbines (VAWT). The blades of a VAWT are arranged vertically. VAWTs has design advantages including the generator being on the ground level for easier maintenance and avoiding the need to change the blade direction every time the wind changes directions. The most related wind turbines to the invention are the vertical axis turbines VAWT including turbines using Darrieus related technology. Darrieus technology invented by D.J.M. Darrieus comprise of curved blades connected at two points along a rotatable tower. When the turbine rotates, the centrifugal forces are reduced with the blades already bulging outward in a shape known as troposkein before the rotor started spinning. The troposkein shape is similar to the shape a rope would take if it was spun around an axis. As a result, the troposkein shape minimizes stresses due to centrifugal forces. U.S. Pat. No. 1,835,018 has a more detailed information of the Darrieus turbine invented by D.J.M. Darrieus.
In generating large amounts of power, conventional turbines had large rotors in order to generate a sufficient amount of energy in order to make it worthwhile for having a generator in order to produce electricity. Unfortunately, the large rotors are expensive because the stress on the rotors increase dramatically as the diameter increases. Conventional turbines had to increase the diameter of the blades in order to capture more energy by increasing the area of moving air which are impacting on the blades. This increase in the diameter of blades for producing substantial power can increase the cost of other items in the turbine other than the blades. Large blades which have not been properly produced can create structural stress and fatigue problems for the gearbox, tower, and the system that turns the generator toward the optimal wind direction.
In the past, wind turbines were supported by a single tower and guy wires in many cases leading to many vibration and frequency related problems. The blades of vertical axis turbines were large and could be limited in the design and the materials used. For example aluminum extrusion and fiberglass pultrusion were used in the two most serious commercial applications of vertical axis turbines. Due to the large size of the fiberglass blades, the strength was limited in order to bend the blade at the place of installation. The aluminum blades could not form a true troposkein shape. The blades had to be made of significant length and the available extrusion equipment is not available. The patents of both serious commercial prior applications of vertical axis technology are described in “Vertical Axis Wind Turbine” U.S. Pat. No. 4,449,053 and
“Vertical Axis Wind Turbine with Pultruded Blades” in U.S. Pat. No. 5,499,904. However, the fatigue factor in blades using those material suffered from structural stress caused by cyclical loads on vertical blades. The lift forces push the blades back and forth as they rotate. The more popular horizontal wind turbines are not subject to this cyclical stress occurring many thousand of times per day. The construction and installation was complex and costly.
The vertical blades in prior technology could not place the rotor high enough above the ground in order to a turbulence leading to long term structural problems
In other prior technology, the swept area of the turbine had an aspect ratio of less than four due to construction limitations. The aspect ratio, the swept area height to diameter, is preferred to be high for better efficiency. This occurs when a tall and thin rotor maintains a large swept area and a high RPM. As a result, the moment of inertia is reduced and less energy is spent on its own motion.
In prior blade technology, two or more blades per shaft section was used in order to achieve proper blade balance. The designing of one blade per shaft section was expensive and had imbalance problem in past turbines there were numerous attempts toward developing a horizontal one bladed turbine. However, it was not seriously commercialized.